1. Field of the Invention
The present invention relates to a printer and in particular to a printer of serial type in which printing is carried out while moving the type wheel in the direction of line. More particularly, the present invention relates to a printer with which printing in different colors is possible employing ink rollers.
2. Description of the Prior Art
In the art there are known printers in which characters can be printed on a common paper and different kinds of data can be printed in different colors. An example of such printer is a printer associated with an electronic table calculator in which data of addition and data of subtraction are printed in black ink, and red ink respectively, so as to make the former data distinguishable from the latter.
However, most of the known printers capable of two color printing use ink ribbons in the form of a tape. This ink ribbon system involves some drawbacks. Firstly, it needs a separate driving apparatus to drive the ink ribbon. Secondly, it is large in size because of the provision of the separate ink ribbon driving mechanism. Thirdly, the driving apparatus increases the consumption of electric power in the printer.
To obviate the above drawbacks of large size and high power consumption there has already been proposed a printer of so-called ink roller type. In the roller type of printer an ink roller containing ink is used. The ink roller is in pressure-contact with a type wheel carrying a number of types on the circumference of the wheel. When the type wheel is rotated, the ink roller also rotates depending on the rotation of the type wheel. During the rotation, the ink roller applies ink to the types on the wheel. A hammer or the like strikes the type wheel through a printing paper to effect printing.
For this ink roller type printer it is no longer necessary to provide a particular driving mechanism to drive the ink roller. Since the type part of the type wheel can be formed of elastic material, the hammer need not have large impact force. As a result, the power consumption in the printer is substantially reduced as compared with the above ink ribbon type printer.
However, unlike the ink ribbon system, the ink roller system has not been useful for two color printing. As described above, in the case of the ink roller type printer, ink is directly applied onto the types on the type wheel and a printing paper is impacted against the types by a hammer or the like. If two different colored inks are applied to one and same type one after the other, no acceptable printing may be obtained. Therefore, it is common knowledge that two color printing is difficult to do employing the ink roller type system. For this reason, prior to the present invention, man has been compelled to use an ink ribbon type of printer in an electronic device which is designed for two color printing although it has the disadvantages of large size and large power consumption.